The present invention generally relates to a permanent magnet having alternately continuously formed N and S poles and an apparatus for producing such a permanent magnet. The present invention particularly relates to a permanent magnet which has a non-magnetic flux region or a weak magnetic flux region between adjacent poles and which is suitable for use in an armature or a field system of an electric motor, and relates to a magnetization apparatus for producing such a permanent magnet.
Conventionally, an electric motor has been a typical example of an apparatus having a permanent magnet constituted by alternately continuously formed N and S poles. In a DC motor, for example, a cylindrical or disc-like permanent magnet has been used for a field system or an armature of the motor. In this case of the cylindrical or disc-like permanent magnet having, for example, four magnetic poles, those magnetic poles are distributed in such a manner as shown in FIGS. 1A and 1B respectively, with circumferential field intensity as shown in FIG. 2.
Since there is no non-magnetic flux region or no weak magnetic flux region provided between adjacent magnetic poles as apparently seen from FIG. 2, it has been difficult to improve the efficiency and characteristics of electric motors.
FIG. 3A shows a typical example of a magnetization apparatus for producing the conventional permanent magnet having four poles as shown in FIG. 1A, and FIG. 3B shows the operation of the magnetization apparatus of FIG. 3A. In FIG. 3A, the magnetization apparatus is constituted by a magnetizing yoke 20 preferably made of a ferromagnetic material and a magnetizing coil 22 connected to a DC power source 26 through a switch 28. The reference numeral 24 designates an object of a magnetic material to be magnetized (hereinafter simply referred to as "magnetized object"), the magnetized object being, for example, cylindrical. The coil 22 is wound around each of projection portions 20a to 20d of the magnetizing yoke 20. A direct current is made to instantaneously flow in the coil 22 so that magnetic flux flows, for example, from the projection portion 20b into the projection portion 20a through the magnetized object 24 as shown in FIG. 3B. As a result, magnetization is performed on the magnetized object 24 in a manner so that S and N poles are formed in regions 24a and 24b of the magnetized object 24 respectively. The field intensity formed by the magnetization is distributed as shown in FIG. 3C. As seen from the drawing, no non-magnetic flux or no weak magnetic flux region exists.
In order to provide a weak magnetic flux region between adjacent poles, therefore, there have been conventionally known such permanent magnets as shown in FIGS. 4A and 4B. FIGS. 5A and 5B and FIGS. 6A and 6B, in which a plurality of permanent magnet pieces separated from each other and each having one pole are stuck to each other through adhesives, metal fittings, or the like.
Those permanent magnets constitute armatures of electric motors. FIGS. 4A and 4B show an armature in which permanent magnet pieces are fixed by using pressing metal fittings. FIG. 4A is a cross sectional front view taken on line A--A of the side view of FIG. 4B. In this case, permanent magnet pieces 2 are fixed on a yoke 8 by pressing metal fittings 4 having screw portions. The reference numeral 6 designates a rotary shaft. In this case, although the pressing metal fitting portions form weak magnetic flux regions, there has been a disadvantage in that it is troublesome to fix the permanent magnet pieces by means of the pressing metal fittings so that the working efficiency is poor to thereby increase the cost. FIGS. 5A and 5B and FIGS. 6A and 6B show armatures in which permanent magnet pieces are fixed by a tie wire and a thin stainless steel pipe respectively. FIG. 5B and 6B are cross sectional side views taken on lines A--A of the front views 5A and 6A respectively. In FIGS. 5A and 5B and FIGS. 6A and 6B, permanent magnet pieces 2 are fixed onto yokes 8 by a stainless steel tie wire 10 and a thin stainless steel pipe 12 respectively. In either case, although the stainless steel portions between the magnet pieces form weak magnetic flux regions, there has been a problem in that fixing work is troublesome to thereby increase the cost and in that the magnetic force of the permanent magnet is reduced because of using the tie wire or the pipe and the distance between the armature and a field system is increased because of the thickness of the tie wire or the pipe to thereby further reduce the magnetic force so that the performance of the armature is reduced. Further, there has been possibilities that adhesion portions are deteriorated by time aging or fixing portions are loosened by vibrations to cause such a trouble that permanent magnet pieces come off from the yoke.